Stringed instruments have been around for over 1000 years. The earliest stringed instruments seem to have developed from tightly strung bows that were used as weapons. Modern acoustic stringed instruments all take mechanical vibrational energy originating in the plucking, strumming, hammering, or bowing of strings, and couple that vibrational energy to the surrounding air. Typically, an acoustic sound board and acoustic resonant cavities are employed to both shape the amplitude-frequency spectrum of the energy coupled from the strings to the air, and to serve as an impedance matching mechanism to efficiently transfer a larger percentage of the vibrational energy of the strings to the air.
Typically, larger sound boards and larger acoustic resonant cavities must be employed to efficiently couple lower frequency vibrational energy from strings into the surrounding air. The familiar piano and double bass play the lowest notes of any stringed instruments in a typical symphony, and their familiar large size stems from the need to efficiently couple the energy of low-frequency string vibrations to the surrounding air.
It is an object of the present invention to provide acoustic stringed instruments of reduced size, which provide improved low-frequency of string vibration to the air, thus providing deeper, louder, richer sound in a compact instrument.
FIG. 1 shows a cutaway drawing of a typical modern acoustic guitar.
When the strings are plucked or strummed, their vibration is transferred to the top of the guitar (also referred to as the sound board) through the bridge. The strings act as mechanical traveling-wave resonators. FIG. 2 depicts sequential xe2x80x98snapshotsxe2x80x99 of the oscillation of a string after it is plucked.
In snapshot xe2x80x98axe2x80x99 in FIG. 2, the string is stretched taught with its center displaced by the plucking or strumming object (finger, pick, etc.). In snapshots xe2x80x98b,xe2x80x99 through xe2x80x98fxe2x80x99 the string vibrates to its opposite phase. Snapshots xe2x80x98hxe2x80x99 through xe2x80x98lxe2x80x99 show the return phase of the oscillation. The sequence then repeats. FIG. 2 shows the oscillation of one standing wave section (half-wavelength) of a vibrating string. Any number of half-wavelength standing wave sections may exist on a vibrating string, allowing one vibrating string to contain vibrational energy at a fundamental and all of its harmonics. Different harmonics on a vibrating string are depicted in FIG. 3.
Vibrational energy from the strings couple to vibrational modes on the sound board, and to Helmholtz resonator vibrational modes of the partially enclosed air volume of the body of the guitar.
FIG. 4 shows Chlandi patterns indicating the resonant modes of the top (sound board) of a guitar at various frequencies. These resonant modes are excited through the bridge of the guitar by the vibration of the strings. Since the sound board has a bigger surface area than the strings, in more effectively couples sound to the air. The sound board acts as a coupled resonator (coupled to the strings through the bridge), and an impedance matching system to efficiently transfer vibrational energy from the strings to the air.
The lowest resonant mode of the sound board on a guitar doesn""t produce any Chlandi pattern, because the only node contour is the outer edge of the sound board itself. In this mode the top (sound board) of the guitar bows back and forth as depicted in cross-section in FIG. 5B. FIG. 5A shows how the sound board would bow back and forth if it were of uniform compliance (such as the membrane of a drum).
Because of the nature of how the edges of the sound board are fixed in a typical guitar, the attachment perimeter is typically more stiff than the middle of the sound board, resulting in the flattened edges of the excursions of the sound board in FIG. 5B as compared to FIG. 5A. The area between the dotted and solid lines in FIG. 5A and FIG. 5B can be thought of as representing the amount of air moved by the sound board resonance. More air moved (more area between the dotted and solid lines) represents a louder sound. In a typical sound board (represented in FIG. 5B), only the middle portion of the sound board has significant excursion in the lowest mode, thus even though a guitar might be xe2x80x9cfull sizedxe2x80x9d, only a fraction of the area of its sound board is making most of the sound.
In addition to the lowest vibrational mode of the sound board, the acoustics of the guitar include another low-frequency coupled resonator which comes from the Helmholtz resonance of the partially enclosed air in the body of the guitar. The air in the body acts as a spring, and the air around the port acts as a mass. The elements of this resonator are depicted in FIG. 6.
All of the resonant modes of the sound board and the air in the guitar will be referred to in this patent application as Coupled Acoustic Resonances. The resonances of the strings themselves will be referred to as Driving Acoustic Resonances.
The present invention allows for a lowering of the two lowest coupled acoustic resonances of sound-board-equipped stringed instrument such as a guitar. Instead of attaching the sound board rigidly around its perimeter, the present invention provides for a sprung suspension around the perimeter of the sound board. The sprung suspension is acoustically sealed so that it does not act as an additional port. A cut-away diagram of a preferred embodiment of the improved guitar body including sprung suspension is shown in FIG. 7. The spring suspension comprises a wooden bellows-like structure connecting the side walls of the guitar body to the sound board.
This bellows-like suspension allows the entire sound board of the guitar to move up and down in a resonant manner. In the present invention, the motion of the sound board in the lowest mode is more like the motion shown in FIG. 5C, whereas the motion of sound boards of guitars previously known in the art is more like the motion shown in FIG. 5B. The compliant mounting of the perimeter of the sound board in the present invention allows a larger fraction of the area of the sound board to take part in close to the full excursion of the center of the sound board. Thus a small guitar made with the compliantly mounted sound board of the present invention can produce a lowest-mode sound volume similar to a larger guitar made by techniques previously known in the art.
The resonant frequency of the compliant suspension in combination with the mass of the sound board may be designed to be much lower than the resonant frequency of the lowest Chlandi mode shown in FIG. 5, and may be designed to be lower than the Helmholtz resonant mode shown in FIG. 6. As with any couple resonators, if the resonant frequency of the compliantly suspended sound board is placed close to the Helmholtz resonance, the frequencies of the poles due to each will be moved somewhat by the presence of the other resonator. The frequency response of the combined resonators may be designed to be broader and flatter in the center of its pass band than either resonator alone. This added resonant mode allows for a small guitar which is richer sounding and more acoustically like a larger guitar.
In an alternate embodiment, another area of the surface of the guitar (besides the sound board) may be compliantly suspended in addition to or in place of the compliant suspension of the sound board. For instance, all or a portion of the back of the guitar may be compliantly suspended. This in effect creates a second sound board which is designed to couple sound energy to the air primarily in its lowest resonant mode.